This application is the national phase under 35 U.S.C. xc2xa7371 of prior PCT International Application No. PCT/EP97/03970 which has an International filing date of Jul. 23, 1997 which designated the United States of America.
This invention refers to an optical module for access networks to wide band communication systems composed of passive optical networks PON, in such a way as to permit the transportation in particularly efficient manner of flows of numeric and/or analog information, relating to different types of services such as:
telephone services in a broad sense, intended as traditional telephone services and data transmission services;
services of more strictly television nature such as the distribution of the CATV.
The access network is commonly identified by one or more connecting interfaces (of type V) in power-station and by various types of interfaces (of type S) for the user terminals, normally placed end to end to apparatuses positioned near the users.
The apparatuses which are used for the above purpose in the access network, and which form the so-called PON access system, are represented by a line termination (Optical Line Termination, or briefly OLT), generally placed or positioned near a switching station, and by a plurality of network units (Optical Network Units or briefly ONU), typically placed near the users, connected to the same OLT through a passive optical network, according to a layout like that illustrated in FIG. 1.
The passive optical network, which forms the so-called access network infrastructure, has typically tree or point-multipoint structure, where the root is connected to the OLT and the terminal branches to each ONU, and is composed of pieces of optical fiber interlinked by passive optical components, called power splitter/combiners, which make it possible to split the optical power between the outputs or recombine the optical signals present at the inputs.
The operators of the above mentioned wide band communication systems have assumed a plurality of methods through which to link the single residential users to the PON network.
One of these methods provides, for example, that the passive optical network reaches a pavement or a building, and, after having made a conversion of the optical signals into electrical signals, the latter are sent to the single users, for example through an equal number of pieces of coaxial cable.
A second method provides instead that the passive optical network is extended until it reaches the single residential users and the invention is preferentially applied when this second method is adopted. The optical module in question is indicated in FIG. 1 with MO and has the function of sending:
to a first termination unit NT_T the signals which transit on the PON relating to the above mentioned telephone services;
to a second termination unit NT_V the signals which transit on the PON relating to the above mentioned television services.
As known, the transmission systems on optical fiber predominantly use predetermined xe2x80x9cwindowsxe2x80x9d or bands of the optical spectrum through which the transmission of the signals along the fibers takes place with a minimum attentuation.
Carrying signals or communication channels, each with its own precisely defined wavelength, as produced by a relevant laser generator, included in one of these privileged windows, or bands modulable in intensity (commonly in digital or analog mode), may be transmitted along an optical fiber with extremely low losses:
The simultaneous transmission of various communication channels belonging to a certain band, or window, or channel on a same fiber, is made possible by operating in Wavelength Division Multiplexing (WDM).
Therefore, in the present context, the term xe2x80x9cchannelxe2x80x9d indicates a certain band of the optical spectrum or xe2x80x9cwindowsxe2x80x9d, used for the transmission of wavelength division multiplexed (WDM) optical signals.
In other words, the term optical band indicates a continuous wavelength interval which may house various optical channels, for example one or more channels for data transmission, one or more channels of telephone type, one or more channels of television type with simple or high definition etc.
In relation to the above, communications of a certain type such as telephone communications by voice or by data, maybe defined between 1260 and 1350 nm. With the same logics, the so-called 3rd useful window, whose passband goes from approximately 1480 to 1580 nm, may be destined or reserved for video transmissions via cable.
Therefore a certain transmission system with optical fibers, operating with carrying signals with a wavelength included in the passband of a first channel or window, whose central or main wavelength is xcex1 (for example 1310 nm), may also support transmissions made in a second channel or window, whose central or main wavelength is xcex2 (for example 1550 nm).
At each user the problem arises of separating the signals transmitted in the 1310 nm band from the signals transmitted in the 1550 nm band.
When the operator provides the allocation of digital signals of telephonic type (for example for traditional and/or data telephony) in the 1330 nm band and the signals for diffusive television in the 1550 nm band, the installation is required on each user""s permises of the above mentioned optical module MO which is illustrated in detail in FIG. 2.
As required by some operators, this module includes:
a device named WDM (Wavelength Division Multiplexing) to whose input is connected an optical support 1 on which transit both the signals allocated in the optical band xcex1 and the signals allocated in the optical band xcex2 which is therefore suitable to
i) make available at a first output the optical signals allocated in the optical band xcex1, that is allocated in the 1310 nm band;
ii) make available at a second output the signals allocated in the optical band xcex2, that is allocated in the 1550 nm band;
an optical power splitter/combiner suitable to split in practically equal manner on two output branches the optical power associated to the input signal;
an RIV detector suitable to supply a level signal proportional to the optical power which crosses the fiber connected to one of the output branches of the optical power splitter/combiner 2;
a laser diode LA_D at 1310 nm suitable to send on the other branch of the splitter/combiner optical signals derived from the digital signals of telephonic type, which, as known, need a transmission of bi-directional type;
a further optical support 3 connected to the second output of this WDM which presents such a radius that the output direction is at 180xc2x0 with respect to that of the input in order to provide the presence of all the interlinking organs on the front side of the optical module MO.
Since, as mentioned above, there is one optical module for each user, its cost must be very limited in order not to be an obstacle for the diffusion of the multimedial services in question.
The most promising technology for the purpose of limiting the costs is the xe2x80x9cglass on siliconxe2x80x9d technology, but it should be borne in mind that the use of this technology is marked by the present of some steps of the production process which need rather long execution times, and in particular one of these steps needs a rather long stay inside special furnaces of the sublayer of silicium for the laying of the layers of glass forming the waveguides.
To permit a better appreciation of the advantages deriving from the precepts indicated in this invention, it is pointed out that the cost of these furnaces is quantifiable at approximately 1 million US dollars for each set of furnaces necessary for the purpose.
The limitation of the production costs of the optical module in question may therefore only be achieved if it is possible to limit its dimensions from which to be able to construct on a single wafer of silicium a very high number of optical modules.
As indication it is pointed out that the usual dimensions of this module are today approximately 5÷30 mm and therefore for production volumes up to several millions of modules/year large investments are necessary, in particular correlated to the cost of these furnaces.
Since, however, as mentioned above, the above mentioned piece of waveguide has a radius of around 180xc2x0, making use of usual phosphor-doped glass to construct the above mentioned optical support or wave guide 3, if the radius is lower than a predetermined amount, the optical losses become unacceptable.
For example, the compliance with the above mentioned width of 55 mm imposes that the radius is not over 2 mm, while, making use of the normal phosphor-doped glass, the minimum obtainable radius is equal to approximately 10 mm. Making use of this type of glass, reducing the curve with respect to this value, the losses become unacceptable.
In order to obtain the limitation of the above mentioned dimensions, it has been suggested to abandon the approach aimed at the reduction of the above mentioned radius and to achieve this objective by making use of a particular WDM*, inside which, in suitable position, is placed a dichroic interferential filter 4, that is a reflecting element as illustrated in FIG. 3 relating to the xcex2 at 1550 nm.
As known, this interferential filter is composed of a glass on which are laid plurality of layers of oxides which give it the property of being transparent to the optical band xcex1 and of reflecting instead the optical band xcex2, which is then coupled to the second output of the WDM* unit.
This WDM* unit is composed of a section of waveguide which runs parallelly to the wave guide of input 1 for a length L/2, where L is the length of the coupling necessary to determine the transfer of all the optical power from the waveguide 1 to the waveguide 2.
The waveguide constructing this section of length L/2 is in practice continued to construct said piece of wave guide which transports the optical band xcex2 on the side of the module provided to support the connectors.
In reality, the waveguide which corresponds to the output of the WDM* unit is cut transversally, usually through an operation of cutting or incision or of excavation, then the above mentioned interferential filter is positioned in the furrow thus obtained.
The optical module constructed according to the known art fully achieves the objective of the above mentioned dimensional limitation, but the relevant production process presents the following problems.
The cutting operation may be carried out with fairly modest tolerances (xc2x1100÷50 xcexcm) which involve considerable processing waste correlated to the non-compliance with the above mentioned length L/2 of the WDM unit.
The cutting operation must be followed by a costly lapping operation to polish the end of the interrupted waveguide.
The positioning of this interferential filter must be made by hand, module by module, using expensive equipment for the automatic positioning which annuls the advantage of reduced cost deriving from the reduced dimensions and of the resulting low investment in equipment for the manufacture of the optical guide.
To sum up, the module constructed according to the invention is a module of hybrid type constructed in part with integrated optics (waveguide) and in part with traditional microoptics (interferential filter).
The use of particular glass doped with germanium which has the intrinsic power of strongly guiding the light is also described in literature.
The use of this technology involves, however, high investments in the purchase of furnaces which deposit glass then destined to be submitted to a doping process using the germanium as doping element.
The invention necessary for the purchase of the relevant equipment for the production of glass doped with germanium is, however, similar to that for the production of phosphor-doped glass and estimable at around one million US dollars.
The object of this invention is to identify an optical module which does not need the use of microoptical component of any type, such as the above mentioned interferential filter.
Another object is to identify a production method which makes it possible to obtain the above mentioned dimensional limitation using only integrated optical components.
A further object is to construct the above mentioned optical module without the use of special glass, such as the glass doped with germanium mentioned above.
This object is achieved by means of the optical module for access networks to wide band communication systems of the type including at least:
a first multiplexer in wavelength suitable to receive in input an optical support, coming from a predetermined side of the module, in which transit signals allocated in a first and in a second optical band and suitable to make available on a first output the signals allocated in this first optical band and on a second output the signals allocated in this second optical band, and
a second optical support, one end of which is adapted to correspond to a predetermined side of the optical module;
a reflecting element placed downstream from one of the optical branches of the first multiplexer in wavelength,
characterized by the fact that
to the second output of the first multiplexer in wavelength is connected a second multiplexer in wavelength of the propagating and counterpropagating type incorporating the reflecting element, the other end of the second optical support being connected to the output of the second multiplexer in wavelength.
An additional object of the present invention is a method for the production of an optical module for access networks to wide band communications systems of the type including at least
a first multiplexer in wavelength suitable to receive in input a first optical support, coming from a predetermined side of the module, in which transit signals allocated in a first and in a second optical band and to make available on a first output the signals allocated in the first optical band and on a second output the signals allocated in the second optical band, and
a second optical support, one end of which is adapted to correspond to the predetermined side of the optical module;
a reflecting element placed downstream from one of the optical branches of the first multiplexer in wavelength,
characterized by the fact that it provides the steps of:
laying on a single water of silicon optical supports forming the first multiplexer in wavelength, the optical support and a second multiplexer in wavelength of the propagating and counterpropagating type;
performing a structure of definition of an area of attack of the ends of the optical supports forming the second multiplexer in wavelength defining it photolithographically by means of a masking process;
chemically attacking the area not involved in the masking with an attack technique of the glass used to construct these optical supports until they are totally removed in such a way as to verticalize their front face;
laying on said front face of the optical supports a layer of material reflecting the light.